Hematopoietic stem cell transplant (HSCT) is a treatment for pediatric patients with a wide range of once incurable and fatal immunological, hematological and oncological diagnoses. The first 6- to 7-months post- HSCT is a critical period during which children are most at risk for life-threatening infections and graft versus host disease (GVHD). To prevent these serious complications, an intense prophylactic regimen is prescribed. Although initially administered in the hospital, after discharge children and their caregivers must manage this complex outpatient regimen, including multiple oral and intravenous (IV) medications. Despite the critical nature of this regimen, our pilot study shows that within the 1st month of discharge, 27% of the oral medication doses prescribed to children were not taken. At 3-months post-discharge, non-adherence (NA) rates climbed to 39%. In fact, NA increased for 74% of the children during this 3-month post-discharge period. NA results in suboptimal drug exposure which increases the risk of GVHD and the more recent the HSCT, the greater the risk. Scientifically, NA compromises conclusions made concerning the efficacy of experimental HSCT protocols in curing rare, life-threatening diseases and the associated toxicities. Despite this, there are no rigorous scientific studies documenting the modifiable factors that predict NA or how NA impacts HSCT outcomes. Therefore, the purpose of the proposed study is to test a new predictive model of adherence in pediatric HSCT. Two hundred and fifty-four children (ages d 10 years) and their caregivers will be recruited and complete online questionnaires to assess factors expected to be associated with NA once prior to the child's discharge and at 1-, 3-, 5-, & 7-months post-discharge. Oral and IV medication adherence trajectories will be systematically documented via objective electronic monitoring and health outcomes and health care utilization will be accessed via electronic medical records. The study aims are to test: 1) a model comprised of clinically relevant pre-discharge caregiver (distress, problem-solving, efficacy and engagement) and child (distress, adaptive skills) factors as well as post-discharge regimen factors to predict outpatient regimen adherence trajectories for 7-months following discharge, 2) the dynamic relationships between post-discharge caregiver, child and regimen factors and outpatient regimen adherence trajectories for 7-months following discharge, and 3) the impact of NA on post-discharge morbidity (i.e., infections and GVHD) and heath care utilization (i.e., re- hospitalizations and emergency department visits). This study will transform current adherence management practices in the pediatric HSCT population by informing the development of assessments to identify children and families at risk for NA before they assume responsibility for the medication regimen, informing the development of practice guidelines, and increasing the precision of medication management. Finally, this study will provide the fundamental knowledge that is needed to develop time- and treatment-specific behavioral and systems-based adherence interventions that will optimize the delivery and efficacy of medical treatments. PUBLIC HEALTH RELEVANCE: For the first time, this prospective study will examine non-adherence to the outpatient medication regimen in children receiving hematopoietic stem cell transplants (HSCT). The results of this study will directly inform the development of practice guidelines regarding the medical monitoring and management of adherence, adherence assessment tools, and adherence promotion interventions. Furthermore, this study will be the first to examine potential links between non-adherence and health outcomes and the health care utilization of children receiving HSCTs. The proposed study is considered to be of high priority for the fields of pediatric HSCT and adherence. More importantly, it is of the highest priority for the children and caregivers who struggle to manage this complex, life-saving treatment regimen with little or no empirically-supported prevention or intervention programs provided to assist them.